Such filters are generally comprised of a fibre or expanded filter medium surrounded by a kraft or cardboard frame. Conventionally the frame is a single sheet of material which is folded around the filter medium and secured at its ends to retain the filter medium. Such a conventional filter is described in Dow et al., U.S. Pat. No. 2,965,197, which I hereby incorporate by reference.
One problem has been to secure the ends of the frame by a means which is both simple and inexpensive, and yet reliable.
Tab and slot arrangements, such as shown in Schnittker U.S. Pat. Nos. 2,952,342 and Champlin 4,086,071, have been used in filters in the past. But such arrangements are often difficult to assemble and do not remain secure.